1. Field of the Invention
The present invention relates to a wireless mobile communication system. More particularly, the present invention relates to a method for effectively transmitting and receiving a Channel State Information-Reference Signal (CSI-RS) and measuring interference.
2. Description of the Related Art
Current mobile communication systems are developing, beyond the initial level of providing voice-oriented services, into a high quality wireless packet data communication system in order to provide a data service and a multimedia service. To this end, various standardization organizations, such as the 3rd Generation Partnership Project (3GPP), the 3GPP2, and the Institute of Electrical and Electronics Engineers (IEEE), are preparing 3rd generation evolution mobile communication system standards employing multiple access schemes using multi-carriers. Recently, various mobile communication standards, including Long Term Evolution (LTE) of the 3GPP, Ultra Mobile Broadband (UMB) of the 3GPP2, and 802.16m of the IEEE, have been developed in order to support a high speed-high quality wireless packet data transmission service based on a multiple access scheme using a multi-carrier.
The current 3rd generation evolution mobile communication systems, such as LTE, UMB, and 802.16m, are based on a multiple carrier multiple access scheme, employ multiple antennas based on a Multiple Input Multiple Output (MIMO) scheme, and use various technologies, such as beam-forming, Adaptive Modulation and Coding (AMC), channel sensitive scheduling, and the like. These technologies improve the system capacity performance by, for example, concentrating a transmission power of multiple antennas or controlling the quantity of data transmitted from the antennas according to the channel qualities and selectively transmitting data to a user having a good channel quality. These techniques are based on the channel state information between a base station or an evolved Node B (eNB) and a mobile station or a User Equipment (UE). Therefore, an eNB or a UE needs to measure the channel state between them, and a CSI-RS is used in the measurement. The eNB refers to an apparatus for downlink transmission and uplink reception, which is located at a predefined position, and one eNB performs transmission and reception with respect to cells. In one mobile communication system, a plurality of eNBs are geographically scattered and each eNB performs transmission and reception with respect to the cells.
A reference signal is a signal used for demodulation and decoding of a received data symbol by measuring channel states, such as the intensity or distortion of a channel, the intensity of interference, Gaussian noise, or the like, between an eNB and a UE. Furthermore, a receiver can determine the channel state of a wireless channel between the receiver and a transmitter by measuring an intensity of a signal received through the wireless channel, which has been transmitted with a predefined transmission power by the transmitter. The measured channel state of the wireless channel is used by the receiver to determine a data rate which the receiver will request from the transmitter.
The resources of time, frequency, and transmission power are limited in a mobile communication system. Therefore, an increase in the quantity of resources allocated to a reference signal may decrease the quantity of resources that can be allocated to transmission of traffic channels and thus may reduce the absolute quantity of transmitted data. In this case, although the performances of channel measurement and estimation may be improved, the reduced absolute quantity of transmitted data may rather decrease the performance of the entire system throughput. Therefore, in order to obtain an optimum performance in view of the entire system throughput, a proper distribution between resources for the reference signal and resources for transmission of traffic channels is necessary.
FIG. 1 illustrates transmission of various signals in a Physical Resource Block (PRB) pair in a Long Term Evolution-Advanced (LTE-A) system according to the related art.
Referring to FIG. 1, one PRB pair includes 14 Orthogonal Frequency Division Multiplexing (OFDM) symbols along the time axis and 12 subcarriers along the frequency axis. The 14 OFDM symbols and the 12 subcarriers form 168 (=14×12) Resource Elements (REs), wherein each RE corresponds to a resource having an orthogonality with respect to a neighboring RE. In the PRB pair, a Physical Downlink Shared Channel (PDSCH) used for transmission of traffic data, a Cell-Specific Reference Signal (CRS) transmitted for each cell, a Physical Downlink Control Channel (PDCCH) used for transmission of a control signal, a Demodulation Reference Signal (DMRS) used for reception of a PDSCH, and a CSI-RS used for measuring a downlink channel state and generating channel state information are allocated different REs for transmission. The CSI-RS supported in an LTE-A system can support signals for one antenna port, 2 antenna ports, 4 antenna ports, and 8 antenna ports, and the number of REs allocated in one PRB pair are different according to the number of antenna ports of the CSI-RS as illustrated in FIG. 1.
FIG. 2 illustrates a transmission of a CSI-RS having four antenna ports in one PRB pair in an LTE-A system according to the related art.
Referring to FIG. 2, as indicated by reference numerals 200 and 210, sequences for four CSI-RS antenna ports are spread by orthogonal codes, Code-Division-Multiplexed (CDM), and transmitted to four REs. The sequences for CSI-RS port 0 and CSI-RS port 1 are transmitted using the sequences for CSI-RS port 2 and CSI-RS port 3 and another RE pair. In this way, sequences for a plurality of CSI-RS antenna ports may be transmitted using a plurality of REs. In a case of an LTE-A system, since transmission to a maximum of 8 CSI-RS antenna ports is possible, an eNB can transmit CSI-RSs for a maximum of 8 transmission antennas.
In the case of an LTE-A system, transmission and reception can be performed using CSI-RSs supporting a maximum of 8 CSI-RS transmission antennas to one transmission point as described above. In a case of performing a beam forming transmission using a maximum of 8 transmission antennas, a beam forming gain of a maximum of 9 dB is obtained, so as to improve the Signal to Interference and Noise Ratio (SINR).
Therefore, a need exists for a method and an apparatus for transmitting a reference signal for effective data transmission and reception, measuring interference, and generating channel state information in a MIMO transmission and reception.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.